With the market demands for brightness, color saturation, service life, non-toxicity and environmental protection of the projection apparatus, the type of the light source applied on the projection apparatus is evolved from an ultra-high pressure mercury lamp (UHP lamp), a light emitting diode (LED) to a laser diode (LD).
Currently, the costs of high-brightness red laser diodes and green laser diodes are still high. To reduce the cost, a blue laser diode is usually applied to excite the wavelength-converting material on a phosphor wheel to convert a blue light into a yellow light and a green light, and another part of the blue light is reflected by or penetrates through the phosphor wheel. A desired red light is filtered by a filter wheel, and then combined with the reflected or penetrated blue light to constitute the three primary colors of red, green, and blue required for the projection images.
Conventional wavelength-converting materials of the phosphor wheel are phosphor powders. The phosphor powders are usually gathered and fixed on a turntable of the phosphor wheel by an adhesive. In a common way provided at present, the phosphor powders are distributed in an organic adhesive to form an adhesive mixture, and then the adhesive mixture is coated on the turntable by printing or dispensing to form a wavelength-converting layer. The coating process above is quite convenient; however, compared with the phosphor powder, the heat conductivity of the organic adhesive is relatively low. The organic adhesive, such as silicone, has a heat conductivity of 0.1 watt per meter Kelvin (Wm−1k−1) to 0.2 Wm−1k−1. The phosphor powder, such as yttrium aluminium garnet (YAG), has a heat conductivity of 8.8 Wm−1k−1 to 13.0 Wm−1k−1. When a laser having a high output power (for example, 100 watts), irradiates on the wavelength-converting layer, as a result of the poor heat conduction of the organic adhesive, the heat accumulates rapidly in the wavelength-converting layer and causes a thermal quenching effect, thereby lowering the wavelength-converting efficiency of the wavelength-converting layer. When the wavelength conversion efficiency is lowered, more heat accumulates in the wavelength-converting layer so that the wavelength conversion efficiency is further lowered. This is a vicious circle. The wavelength-converting layer is unable to dissipate the accumulated heat, thereby causing the organic adhesive of the wavelength-converting layer to be deteriorated and burned black, and resulting in deterioration of the wavelength conversion efficiency of the wavelength-converting layer. Eventually, the wavelength-converting layer may lose the wavelength conversion capability due to the vicious circle.
The information disclosed in this “BACKGROUND OF THE INVENTION” section is only for enhancement understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Furthermore, the information disclosed in this “BACKGROUND OF THE INVENTION” section does not mean that one or more problems to be solved by one or more embodiments of the invention were acknowledged by a person of ordinary skill in the art.